1. Field of the Invention
The present invention relates to an apparatus and method for selectively modifying the radiation emitting, reflecting, and absorbing characteristics of an apparatus, and more particularly to modifying such characteristics as a means of controlling the amount of heat emitted in the form of such radiation, with the apparatus and method being particularly adapted for use in a spacecraft or the like where control of thermal radiation is the primary method of temperature control.
2. Background Art
Spacecraft and satellites in orbit undergo heating from internal electrical and electronic instrumentation, as well as from solar and planetary radiation. This heat can be radiated away at infrared wavelengths if the hot components are thermally connected to the surfaces of high infrared emittance. When the solar heating is cut off by occultation, or when the on-board instrumentation is in a dormant state, the loss of heat by radiation may cause excessive cooling of components (such as batteries) which cannot tolerate the resulting low temperatures.
A common approach to solving this problem is the use of a louver system, generated by metallic springs which open the low emittance louver blades at higher temperatures to expose a high emittance surface, and close the blades at low temperatures, to retain heat. The weight and size of louver systems are significant drawbacks for use in space. In addition, there are potential problems relating to the reliability of such louver systems.
With regard to other solutions to this problem, U.S. Pat. No. 3,374,830, O'Sullivan Jr., shows a thermal control panel for a space vehicle. This device uses the electro-optical effects of transparent materials to control the amount of absorption or reflection of incident solar radiation. An electrical potential is applied to modify the characteristics of the material.
U.S. Pat. No. 3,341,274, Marks, shows an electrically responsive light controlling device, and one of the uses of the device described in the patent is on the surface of space satellites to modify the radiation absorbing and reflecting characteristics of the device. There is a liquid between two sheets of glass or other material, and a suspension of dipole particles in the liquid. The orientation of these particles is changed by applying an electric field, thus changing the ability of the device to reflect, transmit or absorb light.
Neither of the above two devices can modulate the thermal emittance of spacecraft surfaces, since they basically operate at visible rather than infrared wavelengths. They would control temperature by modulating solar heating, which is an inadequate method for thermal control of spacecraft in many cases.
A search of the patent literature discloses a number of other devices which are related somewhat generally to this broad area of technology, and these are presented below.
U.S. Pat. No. Re. 16,733, Jenks, illustrates an apparatus where there is a mirror located in an electrolytic bath that is connected to a source of oscillating current. A light source of constant intensity is directed through a lens and at the mirror, with the reflection from the mirror being directed toward a film. By reversing the current, the mirror becomes the opposite pole of the oscillating circuit and particles are alternately plated onto and removed from the mirror to change its reflective capacity. The purpose of this is to produce lights and shades and half tones on the photographic surface on which the reflected light impinges.
U.S. Pat. No. 3,153,113, Flanagan et al, shows a window having two glass plates with a transparent electrolyte therebetween. There is a transparent electrode between the two sheets onto which material will coat when current is passed through the electrolyte. There is a second electrode covering only a minor portion of the area between the two window panels. The light passing through the window can be filtered by changing the density of the coating on the electrode which extends fully between the two panels.
U.S. Pat. No. 3,169,163, Nassenstein, discloses a light valve where there are two electrodes defining a space therebetween, and there are two membranes spaced from each other and located between the electrodes, with the membranes and the electrodes defining three regions. The upper and lower region are filled with a solution, such as a saturated solution of silver nitrate, while the middle region is filled with a colloidal solution. The intensity and direction of a light beam directed on the member is controlled by changing the concentration of a dissolved material in the liquid by applying an electric field to the electrodes or membranes immersed in the liquid. This is essentially accomplished by the migration of suspended particles.
U.S. Pat. No. 3,190,177, Kaprelian, shows what are called "electrochemical shutters". There is a pair of spaced transparent electrodes with an electrolyte therebetween. A source of electric power is connected between the electrodes which are successively plated and unplated in an alternating fashion. It is stated that such a device can be used in aerial cameras, and also other applications such as sound recording, tachistoscopic projection and other uses for light valves or modulators.
U.S. Pat. No. 3,245,313, Zaromb, shows what is called a "light valve". There is a container having two transparent walls with a plating solution located between the walls. The patent relates primarily to the circuit for causing the metal in the solution to plate out onto one of the walls, there is an agent for oxidizing the metal when plated out. The patent relates to certain details of the electrical circuitry. The overall function is to either reflect or transmit the light which is directed toward the light valve.
U.S. Pat. No. 3,443,859, Rogers, discloses a variable light transmitting device in an electrolytic system which utilizes a material having reversably alterable spectral absorption characteristics, which is a material that in one environment or in one set of conditions is transparent or possesses a color and which in other conditions has different light absorbing or transmitting characteristics. There is a pair of transparent or at least translucent electrodes, between which is an electrolytic solution. The current is directed through the electrolytic solution to attain the desired spectral absorption change.
U.S. Pat. No. 3,473,863, Lewis, shows a variable light transmitting window, where there are two sheets of glass, each having a transparent electroconductive coating on the interior surface. There is a plastic inner layer serving as an electrolyte, and containing soluble thallium or lead compounds. The light transmission is regulated by applying an electric potential across the inner layer to deposit an oxide coating of lead or thallium on one of the electroconductive coatings.
U.S. Pat. No. 3,668,106, Ota, shows an electrophoretic display device where there is an electrophoretic suspension layer of finely divided powder located between the electrodes. By imposing an electric field across the suspension, the optical reflective properties of the suspension layer is changed by changing the spatial distribution of the material.
U.S. Pat. No. 3,708,220, Meyers, shows an electro-optical device to control absorption of radiation by windows, display devices and the like by providing a sandwich arrangement of electrodes, between which are two identical layers of transition metal electrochromic compounds separated by a semi-solid highly conductive sulfuric acid gel electrolyte. The electromagnetic radiation absorption characteristics are altered by the influence of an electric field applied thereon.
U.S. Pat. No. 3,712,710, Castellion, shows a solid state electrochromic mirror which operates on generally the same principle as U.S. Pat. No. 3,708,220.
U.S Pat. No. 3,841,732, Marks, shows what is called a "dipolar electro-optic structure", which is generally similar to the earlier Marks patent, U.S. Pat. No. 3,341,274, discussed above.
U.S. Pat. No. 3,971,624, Bruesch et al, shows another electrochromic device where there are two electrodes, an electrochromic layer and an adjacent charge carrying-transmitting insulator layer situated between the electrodes. At least one of the electrodes is deposited on a supporting plate and at least one of the electrodes is transparent. The charge carrier transmitting insulator layer is a good ion conductor and functions to almost completely block the flow of electrons. This is used as an indicator.
U.S. Pat. No. 3,995,940, Guyon et al, shows a luminous display device where there is a light source, a uniform layer of semi-reflecting material of small thickness, a uniform layer of transparent material deposited on the semi-reflecting material, and a reflecting layer deposited on at least part of the uniform layer of transparent material. The reflecting layer is used to form the patterns which are intended to be displayed. An electrolytic solution is provided, and this coats out onto the layer to form the patterns. The device is arranged so that there is constructive interference to maximize reflectance. There is a permanent reflecting surface at the front so that the device is always partly reflecting.